A semiconductor device is provided with a semiconductor region, a gate electrode, a source electrode and a drain electrode. The semiconductor region is formed on a semiconductor substrate surface and includes a first semiconductor portion of a first conducting type, a second semiconductor portion of a second conducting type, a band gap distinct from the substrate's band gap, more than two accumulated semiconductor layers, and junctions between the layers. The semiconductor layers each contain an impurity of the first conducting type. The gate electrode adjoins a heterojunction between the second semiconductor portion and the semiconductor substrate through a gate insulation film. The source electrode is coupled to the semiconductor region. The drain electrode is coupled to the semiconductor substrate.